The instant invention relates to a fuel control system providing steady state fuel flow to a gas turbine engine of a guided missile in direct proportion to engine fuel requirements. The system uses a pulse-width modulated, solenoid-operated fuel metering valve to provide a substantially linear fuel flow rate in response to control signals derived from engine parameters and communicated from a remote processor via fiber optic link.
The prior art teaches the use of solenoid-operated valves wherein fluid flow through the valve is regulated by means of a magnetic plunger which is normally mechanically biased to seat in a valve body orifice and which is cyclically unseated therefrom by energizing a solenoid coil wrapped around the valve body. The resulting pulsating flow creates difficulties, in the form of stall or flameout, when used for supplying fuel to a gas turbine engine requiring a continuous supply of fuel. In U.S. Pat. No. 4,015,426, Hobo attempts to mitigate the fuel flow discontinuities inherent to fuel systems employing such known pulsed solenoid-operated valves by supplying thereto pulsed control signals having a constant frequency but with a variable pulse width. However, the modification of the pulses as taught in Hobo does not, in and of itself, provide for continuous fuel flow, as the valve taught therein continues to operate in a pulsating manner; rather Hobo relies on the external mechanical damping provided by a elastic fuel line acting as a hydraulic accumulator in order to smooth the flow of fuel to the engine. Hobo further teaches the use of a second valve, 180 degrees out-of-phase with the first valve, to double the effective frequency of the fuel pulse to further mitigate fuel flow discontinuity. The use of multiple valves and external damping means nonetheless remain impractical given the space and cost constraints imposed by a guided missile system application.
U.S. Pat. No. 3,523,676 to Barker teaches a fluid control valve wherein a solenoid-operated plunger is offset from the central axis of the valve so as to induce radial vibration when the plunger is cycled into contact with the valve seat. Such radial vibration is used to reduce the undesirable axial rebound inherent in valves which operate in the aforementioned cyclical fashion, whereby greater fuel flow accuracy is obtained. However, the reduction of axial rebound of the plunger correspondingly further defines each fuel pulse generated by the cyclically-operating valve, thereby increasing the fuel flow discontinuities experienced by the engine.
Moreover, the Barker valve contemplates application in a stationary environment, such as a chemical processing plant, where the corrective vibratory action induced by the eccentric plunger is not defeated by external vibratory sources. Thus, the Barker valve would not be effective in an environment which itself is subject to severe vibratory action, such as within a launched missile.
Thus, in short, systems known to the prior art incorporating solenoid based metering valves deliver fuel in a pulsating fashion and require external damping and/or internal vibratory control to stabilize fuel flow to an engine. Systems of this sort are impractical for use in sensitive fuel control applications such as that required for guided missiles in which space and cost considerations, as well as reliability of operation, are critical factors.